Grease Pencil Help - Placing 2D Stroke Animation Over 3D Mesh Object?
I guess it wouldn't be a problem in Blender 2.79. There was an option to keep the grease pencil always on top of 3d objects (it was called x ray i think). In 2.8 You have to be really carefull while chosing position of your grease pencil object, and the mode of drawing (origin, 3d cursor etc.)  I would make the bouncing ball a separate grease pencil object, and animate it's positon first, making sure that the GP object stays on top of the 3d stairs, and then make handdrawn part. Or you could use the the "3d cursor" drawing mode, and make sure that the cursor is on correct place on every keyframe. Or finnaly You could just send the the handdrawn part and 3d staircase to different view layers, and finish it in the compositor (putting the ball animation on top of the stairs) But still, you got the part at the end, where the ball is supposed to be behind the stairs. I would go for the first solution. Not sure if it helps you, but I do some intense research on GP as well and still have to learn a lot about Blender